Supernanograting enables spectrometer sensor

HUNTSVILLE, Ala. – Combining nanophotonics technology with traditional optical spectroscopy has yielded a new kind of optical spectrometer with sensing and spectral measurement functions.
Traditional optical spectrometers measure the spectra of light; traditional optical sensors use light to detect the presence of chemicals. Now, scientists at The University of Alabama in Huntsville have combined the characteristics of both into a single nanoscale photonic device called a supernanograting, demonstrating a new kind of optical sensing apparatus called a spectrometer sensor.

A spectrometer sensor is an optical spectrometer and also a chemical sensor because it measures the optical resonance spectrum that is controlled by chemicals bonded on the nanostructure surface. This device can detect toxins or contaminants in very small quantities.

“Spectrometer sensors are best suited in applications requiring small size and weight,” said Dr. Junpeng Guo. The small size and light weight of the sensors may be useful for NASA space exploration applications like measuring the chemical makeup of the surface of Mars, he said.

Dr. Junpeng Guo, University of Alabama in Huntsville associate professor of electrical engineering and optics, and doctoral student Haisheng Leong view the spectra from a new nanoscale photonic device.
Two spectrometer sensors have been demonstrated recently, one with a super-nanoslit metal grating (Optics Letters) and the other with a supernanohole metal grating (Optics Express).

Optical resonances of nanostructures, a fundamental phenomenon in optics, typically are measured using optical spectrometers. By creating a supergrating pattern of nanostructures, Guo and doctoral student Haisheng Leong made superdiffraction gratings with nanograting structures. With the supernanograting, the resonance of the nanostructure can be measured with a photodetector array, eliminating the need for an optical spectrometer.

The nanostructures first were drawn with a computer and then made using electron beam lithography, which controlled the movement of the tightly focused electron beam to write nanoholes or any other nanostructure pattern in a thin layer of special polymer called e-beam resist.

The e-beam-written polymer layer is then developed so the nanostructure patterns are imprinted to the thin polymer layer. The patterned polymer layer works like a mask, and an argon ion etching process is used to transfer the pattern from the polymer layer to the thin metal film underneath it. This device was made by Leong.

The supernanogratings – a super-period nanohole array drilled in a thin gold film on a transparent glass substrate that supports collective free-electron oscillations, or surface plasmons – have rich physics that need to be investigated, said Guo, an associate professor of electrical engineering and optics. His paper, recently published in Applied Physics Letters (doi: 10.1063/1.4771992), explains the resonance mode splitting phenomenon observed in the supernanohole grating. Such mode splitting could be used to make better-sensitivity chemical sensors.